1. Field of the Invention
The present invention relates to a improvement of a non-aqueous electrolyte secondary cell, and more particularly, to an improvement of positive electrode active material for the purpose of enhancing the cycle life of the cell.
2. Description of Related Art
Non-aqueous electrolyte secondary cells having a negative electrode of lithium or a lithium compound have been expected to have a high voltage and a high energy density, and many researches have been made.
Particularly, an intensive research have been made about MnO.sub.2 and TiS.sub.2 as positive electrode active materials. Recently, Tackeray et al have reported that LiMn.sub.2 O.sub.4 could be used as positive electrode active material, [Mat. Res. Bull., Vol. 18, pp. 461-472, (1983)].
LiMn.sub.2 O.sub.4 has a cubic system crystal structure of the spinel type, and can be used as positive electrode active material in a cell resulting in generation of a high discharge voltage thereof in the order of 4 volts. Thus, it has been expected to be a prospective positive electrode active material.
However, this positive electrode active material produces a problem about cycling characteristics associated with charge-discharge cyclic operation. That is, repetition of charging and discharging results in a considerable reduction of discharge capacity.
The LiMn.sub.2 O.sub.4 positive electrode active material gives rise to a two step-type discharge curve where two flat regions appear at about 4 and 2.8 volts during discharge to 2 volts after charging to 4.5 volts as shown in FIG. 2, wherein the abscissa represents a composition of positive electrode active material used. The charge and discharge are effected with insertion of Li ions into the positive electrode active material and extraction of Li ions therefrom as reported by Ohzuku et al, (Proceedings of the 29th Electric Cell Symposium). The positive electrode active material having a composition of Li.sub.x Mn.sub.2 O.sub.4 causes charging and discharging with a variation of x.
In view of the second flat step region at about 2.8 volts, a good stability of cycling characteristics has been achieved by limiting a charging voltage up to 3.8 volts with discharging voltage being limited down to 2 volts, that is, by the charge and discharge cycles with a variation of x from about 1 to 1.85.
However, such technique cannot achieve a high energy density. In order to achieve a high energy density, the first step cycle wherein charging up to 4.5 volts and discharging down to 3 volts may be allowed, that is, the charge until x reaches 1 or less, preferably 0.7 and the discharge until x reaches 1 or 1.85, may be advantageously employed. However, the cycle where charging is performed until x reaches less than 0.7 exhibits an inferior cycle life in that the discharging capacity is reduced to a half while effecting a number of cycles in the order of only about 50. When charging is performed to an extent as x being over 0.7, an insufficient charge is caused making it difficult to obtain a sufficient discharging capacity.